My Kappa 15LF's are running in 5 cuft boxes tuned to something like 37 Hz. I made them like two years ago so the memory is a bit fuzzy, they're also sold now.The bottom end is quite solid on them. It leaves *just* a bit to be desired, but they are quite smooth and go quite deep. I've played some movies on them and they handle the low rumble surprisingly well.
Adam